1. Technical Field
The present invention relates generally to power supplies. More particularly, the present invention relates to transformers.
2. Background Art
Many electrical devices have a power supply which includes a portable transformer. As is known in the art, a power supply generally includes a transformer, a rectifier and a filter. A popular type of transformer takes the form of a box which may have prongs that plug into a wall outlet, or which may have a cord extending to a wall outlet and another cord extending to the device.
Box-type transformers provide several advantages. Electrical noise is isolated outside the device, rather than including the entire power supply as part of the device. Potentially dangerous voltage is isolated outside the device. In addition, heat from the transformer is isolated away from the device.
However, such transformers also have several disadvantages. With very large box transformers, the heat generated may require a system for cooling. Often, the cooling system utilizes chemical coolants, such as freon, to cool the transformer. Such chemical coolants may be potentially dangerous to the environment. In addition, cooling systems may add to the cost of the power supply and/or the device(s) it is associated with. With smaller box transformers, the major disadvantage is inconvenience. For portable devices, a box transformer can be cumbersome to transport. Smaller box transformers may also be forgotten, rendering the devices useless. Also, plug-in box transformers often fall out of the electrical outlet due to their own weight. In addition, the box-type plug-in transformer may cover up other outlets.
Another type of transformer potentially solves the problems associated with box-type transformers. Transformers shaped like appliance powercords are being re-examined. These transformers do not suffer from the disadvantages associated with box-type transformers. Heat is dissipated along the length of the transformer, rather than being concentrated in one place. Assuming a powercord transformer is attached to a portable device, it cannot be forgotten in transport. As a conventional plug can be used with a powercord transformer, other outlets are not covered up. In addition, since a powercord transformer's weight is dispersed over its length, the possibility of it falling out of the outlet is greatly reduced.
In the prior art, cord-like transformers, hereinafter referred to as powercord transformers, are highly inefficient and may not work. One example of a powercord transformer is described in U.S. Pat. No. 2,436,742, issued to Bussey. Disclosed there is a combination transformer and powercord. However, the Bussey powercord transformer, referred to therein as a line cord transformer, fails to provide a reliable return path for the magnetic flux produced in the core. Presumably, although not disclosed therein, Bussey utilizes air as a return flux path to induce a voltage in the secondary winding. Given that air has a low permeability (.mu.), it is a distinct possibility that no voltage or an insufficient voltage will be induced in the secondary winding. For example, a flux path consisting of iron or various iron alloys has a .mu. of from 50 to 200,000, compared to air which has a .mu. of 1.
Thus, a need exists for a reliable, efficient powercord transformer that does not need a cooling system and is convenient to use.